Method of growing large single crystals



March 24, 1970 H. w. CHANDLER METHOD OF GROWING LARGE SINGLE CRYSTALS Filed July 25. 1967 mzmiwx $5.55 EES .5.6m wzzmr :9.55m wwzwnwxwmkow Mwamtz wgxm 2924 mzomm vom :2545 N ,555V

N wddrrm INVENTOR HORACE mcHnNoLER ATTORNEY5 United States Patent O 3,502,556 METHOD OF GROWING LARGE SINGLE CRYSTALS Horace W. Chandler, New York, N.Y., assignor to Isomet Corporation, Palisades Park, NJ., a corporation of New Jersey Filed July 25, 1967, Ser. No. 655,829 Int. Cl. B01d 13/02 U.S. Cl. 204-180 5 Claims ABSTRACT OF THE DISCLOSURE Large single crystals are grown by introducing cations and anions, for forming the desired crystal, from respective nutrient solutions thereof into a growth solution for supporting the desired crystal growth, under the iniluence exerted by an external source of D-C power. Current flow through the solution is effected by movement of the ions, and is a function of the magnitude of voltage applied from the external source across a pair of electrodes introduced into the overall solution. To prevent any affect upon crystal growth as a result of undesirable ions generated by electrode reaction, the electrodes are immersed in special respective solutions isolated from the nutrient solutions by ion exchange membranes which permit current flow but block the passage of the undesired ions to either the nutrient solutions or the growth solution.

BACKGROUND OFl THE INVENTION The present invention relates generally to the preparation or synthesis of crystals, and in particular to processes for growing large single crystals of unstressed character.

Several techniques are currently employed for the production of large single crystals of the type required in the optical and electronic industries. Many of these techniques and methods are discussed in detail in the literature, for example, The Art and Science of Growing Crystals, I. l. Gilman (Wiley & Sons, New York), 1965, and need not, therefore, be elaborated upon here. It is suiicient to note that methods currently used for the growth of crystals have not enjoyed complete success in the production of certain kinds of crystals. As a specific example, high temperature techniques are unsuitable for growing crystals having a solid-solid phase transformation at a temperature below that used in the growing process, because the phase transition occurring as the crystal is cooled to room temperature introduces severe stresses and consequent cracks in the structure. Conventional solution growth techniques are inapplicable in many instances because the crystal is insoluble, thus precluding the use of programmed temperature or controlled evaporation methods available with highly soluble materials.

In my copending U.S. vpatent applica-tion Ser. No. 612,190, entitled Apparatus and Method for Growing Crystals, led Ian. 27, 1967, and commonly assigned herewith, I disclose certain methods for growing single large crystals by which to overcome at least some of the limitations characterizing the prior art techniques. Briefly, according to that invention, a method of growing crystals is based upon the slow, controlled introduction of cations and anions required to form the particular crystal, from respective source salts into an isolated volume of solvent. The ions are introduced at a rate suiciently slow to prevent supersaturation of the solvent, and thereby excessive nucleation of the solution. The latter, if permitted to occur, would result in the formation of a large number of small crystals rather than the desired large single crystal. As the slow introduction of ions is continued, the ionic concentration begins to exceed the solubility of the ionic crystal in the solvent, at which point crystals start to form ICC in solution. If a seed crystal is introduced into the initially saturated solution, the seed crystal can be caused to grow by this technique.

As disclosed in my aforementioned copending application, isolation of solution in which growth is occurring from the two nutrient solutions which act as the source of the respective cations and anions is accomplished by means of permeselective ion exchange membranes, which are essentially ion exchange resins in membrane form, permeable only to ions. That is, a cation exchange membrane is permeable only to the flow of cations, and an anion exchange membrane is permeable only to the ow of anions. In particular, an anion exchange membrane is used to isolate the growth solution from the anion nutrient solution while a cation exchange membrane is used ot isolate the growth solution from the cation nutrient solution. An electrode is placed in each of the two nutrient solutions and the electrodes are connected to a source of D-C power. The electrode in the cation nutrient solution is connected to the positive (-1-) terminal of the power supply while the electrode in the anion nutrient solution is connected to the negative terminal of the power supply. When a voltage is impressed across the two electrodes, a current flows through the three solutions, i.e., anion nutrient, growth solution, cation nutrient, the current carried by the moving ions. The cations from the cation nutrient solution are attracted to the negative electrode in the anion nutrient solution, but can pass only through the cation exchange membrane, not through the anion exchange membrane. Similarly, anions from the anion nutrient solution are attracted to the positive electrode in the cation nutrient solution, and accordingly proceed through the anion exchange membrane but not through the cation exchange membrane. The rate of introduction of these ions from the nutrient solutions into the growth solution is controlled by controlling the applied voltage, which in turn controls the flow of current through the solutions. The net effect of the process disclosed in my copending application is to introduce only the cation and anion required for the growth of the crystal into the growth solution thereby avoiding the introduction of impurity ions which might affect the growth rate or growth habit of the crystal.

While my previously disclosed process, as briefly described above, is operative to produce single crystals, problems may be encountered, at times, as a result of electrode reaction with the nutrient solutions, problems which can cause premature termination of a run, i.e., of the production of a crystal. In particular, depending upon the specific type of crystal desired, it may happen that undesired ions are generated as a result of electrode reaction and that such ions are transported along with the desired ions, or are mixed with the respective nutrient solution, resulting in termination of the growing process before the desired crystal growth has been attained.

Accordingly, it is a principal object of my present invention to provide improved methods of growing large single crystals. v

It is a more specific object of my present invention to provide methods of growing large single crystals, by which electrode reactions are prevented from undesirably affecting the crystal gro-wth.

SUMMARY OF THE INVENTION Briefly, according to the present invention special solutions are employed in which to immerse them electrodes and these solutions (and therefore, the respective electrodes) are isolated from the nutrient solutions by respective ion exchange membranes. In this manner, any ions generated as a result of electrode reaction may be readily prevented from entering the nutrient solutions or the growth solution, and hence from affecting the crystal synthesis.

It is therefore still another object of the present invention to provide processes for growing large single crystals of any of a wide variety of crystals whose anion and cation components can be supplied by soluble compounds, and wherein the anion and cation nutrient solutions and the growth solution are isolated from any electrode reactions.

BRIEF DESCRIPTION OF THE DRAWING The above and still further objects, features and attendant advantages of the present invention will become apparent from a consideration of a specific example of a process in accordance therewith, especially when taken in conjunction with the accompanying drawing in which the single figure is a schematic representation of apparatus suitable for practicing such a process.

DESCRIPTION OF AN EXEMPLARY PROCESS As an example of a preferred embodiment, a process will be described in which large single crystals of calcium carbonate having the calcite crystal structure are grown in the laboratory. In the past, naturally-occurring large clear crystals of calcite have been found, but attempts to synthesize such crystals on an experimental basis have been largely unsuccessful.

The apparatus schematically illustrated in the tigure has been successfully employed for such purpose, in the following manner. Initially, the assembly shown in the drawing may be constructed using two L sections and 11 and three T sections 12, 13 and 14, all preferably of approximately one inch diameter (or greater) glass tubing. The several glass sections may be clamped together in any suitable and conventional manner, such as by using Teflon (Du Pont trademark) gaskets (not shown) between sections, to prevent intercompartment leakage, the L sections being outermost in the assembly. This arrangement serves to provide ve compartments, designated 15, 16, 17, 18 and 19 in the draw-' ing, each compartment separated from its immediate neighbor or neighbors in a manner to be discussed presently.

Compartment 17, centrally located in the assembly, is lled or partially lled with a growth solution appropriate to the crystals to be formed therein, in this instance a filtered aqueous solution of 0.1 molar ammonium nitrate. The compartments 16 and 18 immediately adjacent the growth solution compartment 17, on either side thereof, are supplied with respective ion nutrient solutions, in this instance the contents of compartment 18' being a cation nutrient solution constituted by a dilute aqueous solution of calcium nitrate, whereas compartment 16 contains an anion nutrient in the form of a dilute aqueous solution of sodium carbonate.

Compartments 16 and 18 are separated from compartment 17, such that their contents are normally isolated from each other, by suitable ion exchange membranes 21 and 22, respectively, the former an anion exchange membrane, which is permeable only to anions, and the latter a cation exchange membrane, which passes only cations.

Electrodes 25 and 26, preferably of platinum screen, are immersed not in the respective ion nutrient solutions as in the invention disclosed in my aforementioned copending application, but in special solutions contained in compartments and 19, separated from the ion nutrient compartments by additional ion exchange membranes 28 and 29, respectively. In particular, in this example chamber or compartment 15 is filled with a dilute solution of sodium hydroxide, and compartment 19` with a dilute nitric acid solution. Electrodes and 26 are connected to the negative and positive terminals, respectively, of a D-C power supply 30, and the electrode solutions in compartments 15 and 19 are isolated from the ion nutrient solutions by cation exchange membrane 28 and anion exchange membrane 29, respectively.

Preferably, the ion nutrient solutions are stirred or otherwise agitated, as by stirrers 33 and 34, during the process of crystal growth.

When a voltage is impressed across the two electrodes from power supply 30, a current flows through the five solutions, i.e., positive electrode Solution, cation nutrient solution, growth solution, anion nutrient solution, and negative electrode solution, as a result of ion motion, the magnitude of the current being proportional to the rate of introduction of ions from the nutrient solutions into the growth solution and being controlled in accordance with the magnitude of the applied voltage. To permit control of the applied voltage, a variable resistance 35 may be placed in series circuit with the power supply and the electrodes.

In response to the voltage applied to the electrodes, cations (Ca-H) from the calcium nitrate nutrient solution in chamber 18 are attracted toward the negative electrode 25 and thereby diffuse or permeate through cation exchange membrane 22 into the growth solution in compartment 17. They are then prevented from further travel by the blocking characteristics of anion exchange membrane 21 to all but anions. Similarly, anions (CO3**) from the sodium carbonate solution in compartment 16 move toward the positive electrode 26 via anion exchange membrane 21, and thereby into the growth solution of calcium carbonate, and can go no further.

Duringr the preceding operation, hydrogen ions (H+) are generated in the electrode solution of dilute nitric acid in compartment 19 and hydroxyl ions (OH) are generated in the electrode solution of dilute sodium hydroxide in compartment 1S, as a result of electrode reaction with the respective solution. However, these ions are prevented from passing into the ion nutrient compartments by the presence of anion exchange membrane 29 and cation exchange membrane 28, respectively, therebetween. Similarly, the ion nutrients cannot enter the electrode solution compartments because of repulsion during energization of the electrodes and because of the respective ion exchange membranes between those compartments. Were this not the case, hydrogen and hydroxyl ion concentrations would be allowed to build within the adjacent nutrient solutions during a growth process, and since these ions are transported more readily than the cation and anion nutrients through the respective ion exchange membranes, the large concentrations would result in eventual preferential transport of hydrogen ions and hydroxyl ions into the growth chamber, thereby causing premature termination of crystal growth. The respective ion exchange membranes between nutrient solutions and electrode solutions are quite effective to prevent this mixing, and permit the process to proceed to a desired completion (both as to selection of rate of growth and of final size of the crystal structure).

In one run in which the process was carried out to produce the aforementioned calcite crystal structure, variable resistance 35 was adjusted such that the voltage applied to the electrodes from source 30 was sufficient to produce a current flow of about l milliampere through the solution. The run was performed at ambient room temperature, and no attempt was made to control the solution temperature. Current was permitted to flow for approximaely 300 hours, after which the large single crystals that had formed in the growth solution were removed and examined. Microscopic examination revealed well formed crystals having the characteristic trigonal structure of calcite. The crystals were about 0.5 millimeter on a side and were found to dissolve in hydrochloric acid with evolution of gas. In all characteristics, i.e., shape, form, crystal habit, and chemical behavior, the synthetically produced crystals were found to be identical to naturally-occurring calcite crystals analyzed by X-ray powder pattern analysis.

The process is general and is applicable to the growth of a wide variety of crystals whose anion and cation components can be supplied by soluble compounds appropriate to the particular crystal desired to -be grown. Variations and modications of the process such as those suggested in the linal passages of my aforementioned copending application are similarly within the scope of the present invention.

Having thus described my invention, what is desired to be secured and protected by United States Letters Patent is:

1. Process for growing large single crystals, which includes migrating cation and anion components of the desired crystal from solutions containing such ions into a growth solution of the desired crystal composition via respective cation and anion exchange membranes normally separating the ion solutions from the growth solution, by energizing a pair of electrodes immersed in solution to govern the lflow of said cations and anions while isolating the electrode solutions from said ion solutions and growth solution to substantially prevent the passage of ions generated by electrode reaction thereto.

2. The process of claim 1 wherein said electrode solutions are located immediately adjacent said ion solutions and are separated therefrom by respective ion exchange membranes.

3. The process of claim 2 wherein the rate of growth of the crystals is determined by selective control of the magnitude of energization of said electrodes.

4. The process of claim 3 wherein said electrode solutions produce ions of the same polarity of charge as said crystal ion components of the ion solutions to which they are immediately adjacent in response to said electrode reaction, the cation and anion solutions being disposed adjacent the growth solution and separated therefrom by said cation and anion exchange membranes, respectively, and the electrode solutions being separated from the cation and anion solutions by further respective cation and anion exchange membranes.

5. The process of claim 4 wherein said large single crystals are crystals of calcium carbonate having calcite crystal structure, said anion solution comprising sodium carbonate, said cation solution comprising calcium nitrate, said growth solution comprising ammonium nitrate, said electrodes composed of platinum, the positively energized electrode immersed in an electrode solution of dilute nitric acid immediatley adjacent said cation solution, and the negatively energized electrode immersed in an electrode solution of dilute sodium hydroxide immediately adjacent said anion solution.

References Cited UNITED STATES PATENTS 1,546,908 7/1925 Lapenta 204-180 1,801,784 4/1931 Schwarz 204-301 X 2,251,083 7/1941 Theorell 204-180 3,231,485 l/1966 Kuwata et al. 204- JOHN H. MACK, Primary Examiner A. C. PRESCOTT, Assistant Examiner Y Us. C1. Xn, 

